A few ventilator questions

[FunnyDocMan1234]

Hi all - would appreciate some clarification on some points of confusion that have come up during the start of fellowship:

• Are ventilated breaths still positive pressure on a low level of pressure support? I assumed they were but then I saw a patient on PS 10/5 whose IVC on ultrasound was clearly collapsing during inspiration, indicating a negative pressure breath. How does this make sense if the vent is giving the patient positive pressure and flow? Finally, does this mean that PS does not have the negative hemodynamic effects associated with other vent modes?

• In PCV mode in a nonparalyzed patient, is patient effort able to increase tidal volumes or are the volumes solely determined by lung and chest wall elasticity? If patient effort cannot increase tidal volumes, how exactly does the vent allow for effort to draw in higher TV in PSV but not in PCV (e.g. is there some sort of valve that gets turned on?)?

• In patients with COPD exacerbation on the vent, the traditional teaching is that you should match the PEEP to the patient’s auto-peep in order to prevent high WOB by the patient to overcome auto peep and trigger a breath. However, if you paralyze the patient is it true that this reason goes out the window and you should minimize PEEP as much as possible?

• I’ve heard PCV is a good vent mode for neuromuscular disease patients. Is this true and if so why exactly is this?

and one non-vent question:

• What do you do if a patient is unable to lie flat or trendelenberg for a central line? I’ve heard some say that you have to do a fem line and others say that as long as the IJ isn’t collapsing on ultrasound it’s safe to put in an IJ in a semi-recumbent position. Is the latter actually safe or if I do this am I risking an air embolism and lawsuit?

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[SurfingDoctor]

If air is being forced in... it's positive pressure. If the muscles are contracting (or the chest is being actively pulled out) creating a negative pleural pressure... it's negative pressure.

As for lines... find the position that makes them the biggest to poke.

 

[Tipsy McStagger]

Are ventilated breaths still positive pressure on a low level of pressure support? I assumed they were but then I saw a patient on PS 10/5 whose IVC on ultrasound was clearly collapsing during inspiration, indicating a negative pressure breath. How does this make sense if the vent is giving the patient positive pressure and flow? Finally, does this mean that PS does not have the negative hemodynamic effects associated with other vent modes?

Yes they're still positive pressure. The hemodynamic effects are dependent on other factors as well (volume status, PVR/SVR, R/L ventricular function etc). The IVC is an imperfect measure, it only tells a small part of the story.

In PCV mode in a nonparalyzed patient, is patient effort able to increase tidal volumes or are the volumes solely determined by lung and chest wall elasticity? If patient effort cannot increase tidal volumes, how exactly does the vent allow for effort to draw in higher TV in PSV but not in PCV (e.g. is there some sort of valve that gets turned on?)?

Yes patient effort will increase the tidal volumes as the set pressure remains constant. If you have them in an adaptive mode like PRVC then the vent will deliver less pressure for the next breath to try and target the goal tidal volume. But in straight PC it will give the same pressure regardless of what the patient is doing.

In patients with COPD exacerbation on the vent, the traditional teaching is that you should match the PEEP to the patient’s auto-peep in order to prevent high WOB by the patient to overcome auto peep and trigger a breath. However, if you paralyze the patient is it true that this reason goes out the window and you should minimize PEEP as much as possible?

If you paralyze the patient then you don't have to worry about patient effort and inspiration triggering, the vent will just trigger. you should titrate your PEEP to each patient accordingly. There are many ways to think about setting PEEP. Caramez has a review on them as well as an interesting paper on setting PEEP in severe asthma exacerbations which has served me pretty well so far.

A comparison of methods to identify open-lung PEEP

Deranged Physiology, always a good resource

Paradoxical responses to positive end-expiratory pressure in patients with airway obstruction during controlled ventilation​

I’ve heard PCV is a good vent mode for neuromuscular disease patients. Is this true and if so why exactly is this?

This is the classic teaching, but I think much depends on why they are intubated and what the goals are exactly. There is no one right setting for all patients.

What do you do if a patient is unable to lie flat or trendelenberg for a central line? I’ve heard some say that you have to do a fem line and others say that as long as the IJ isn’t collapsing on ultrasound it’s safe to put in an IJ in a semi-recumbent position. Is the latter actually safe or if I do this am I risking an air embolism and lawsuit?

Again there's no one right answer. You can put in a fem line, or IJ. If you know how to follow your needle and use appropriate technique you shouldn't cause an embolism, but unfortunate events do happen from time to time. The ICU is an imperfect world, and we have to play the hand we are dealt. We intubate under less then optimal conditions on sick and dying people, we do procedures on the same population. You do what you have to do, always weighing what you stand to gain by what you stand to lose and the harm you may cause. In short, you can put in a line in an upright patient. If shock trauma can cannulate someone for ECMO on a ladder with the patient upright you can put in a CVC in a semi-recumbent patient.

I just hired her, n=1 - starts around 16 min. There's a picture of them cannualting her at 17:33.

 

[leviathan]

1. It depends on which 'chamber' of the body you are referring to as positive pressure: If the airways, then a ventilator always delivers positive pressure to the airways. If the pleural space, then you are seeing it expand both by positive pressure (from the vent) and negative pressure (from the patient's diaphragm contraction). Whether the pleural pressure is negative or positive at the end of the breath depends on which component is delivering more pressure (the ventilator or the patient).

With respect to IVC collapse, this happens when the intraabdominal pressure exceeds the central venous pressure of the IVC. Whether that happens again depends on whether you are doing mostly a positive change in pleural pressure or a negative change (based off contribution of vent vs patient).

2. PCV delivers a set airway pressure to the patient for a set period of time. If the patient is paralyzed, then the delivered tidal volume depends solely on chest and lung compliance and airway resistance. However, if the patient is also breathing spontaneously, things change a bit. For example, if you set an inspiratory pressure of +20, but the patient is generating -5 cm of pressure, then the airway pressure would drop to 15. However this doesn't happen because the ventilator will increase flow further to continue to target that pressure of 20. More flow over the same period of time = larger tidal volume.

3. This is true. AutoPEEP can happen while paralyzed, so you just need to know your true PEEP might be higher than set PEEP, which has all the same benefits and risks of you setting the same PEEP on the vent. The one difference is that if you don't have any limitation in expiratory flow, setting the PEEP higher may decrease your ability to exhale the entire breath. However, most patients with asthma and COPD do have expiratory flow limitation, so it's a moot point.

4. I'm not really sure why PCV would be superior to volume control for these patients. In general, PCV is better tolerated as you are allowing the patient to take as large a breath as they feel like, and as much flow as they want, so they don't have flow starvation.

5. You can do it upright, but you may have a higher risk of injury and air embolism if the vein is collapsed. However if it's a CHF patient or likewise with a big IJ in any position, then I have done them in a non-supine position.